Thank you very much!
It seems that Outlook did not see the network drive - as I changed the $file to "C:/Temp/Text.txt" the email came with the attachment.

The dd [ $message->display ] produced [1] if the email came without attachment, in the last run (as email came with attachment) there were no output (I put the line as the last line in the script).
The Win32::OLE->Option(Warn => 3); produced a message that an exception error in PROPERTYPUT "To" occured at the line 380 of Message.pm. This line deals with the "To" field ($self->{message}->{To} = $self->{To}; ) and it occures even after the successful delivery of the email (with or without an attachment). I have just used my address to test the email.